What characterizes the fully developed stage of a fire?

The fully developed stage of a fire is characterized by all combustible materials within the compartment being involved in the burning process. At this point, the fire has typically reached its maximum heat release rate, and a significant amount of energy is being produced. This stage signifies that the fire has transitioned from the initial phase, where only a few materials might be ignited, to a state where the fire consumes all available fuel.

During the fully developed stage, the conditions within the compartment become critical, often leading to extreme temperatures and smoke production. The fire will continue to grow unless intervention occurs, either through active fire suppression or by a lack of available fuel once it has consumed all combustibles within the space.

This helps to understand why the other responses do not accurately characterize the fully developed stage. If only some materials were burning, it would indicate that the fire has not yet reached full development. Similarly, if the fire is being extinguished or if suppression efforts are ongoing, it is a sign that the fire is not in this critical stage, as suppression efforts would typically occur during or after the fully developed stage is reached.